knowledge

Is aircraft fuel made from sunlight really the future of carbon neutral aviation?

Flying with solar kerosene – the aircraft fuel made from sunlight may sound like science-fiction but it is actually true. And it can mean a real breakthrough in decarbonizing the aviation industry.
Picture of Joanna Kocik
Written by Joanna Kocik
Content Strategist at Oncarbon
May 5, 2022

In March 2022, Swiss and Lufthansa announced a strategic collaboration with Synhelion, with the goal of bringing Synhelion’s aviation fuel to market. The process developed by Synhelion uses sunlight to produce kerosene that, when burned, generates only as much as was removed from the atmosphere in its production. This means that the new “solar kerosene” can make an important contribution to the decarbonization of air transport.

Synhelion announced that it will build the world’s first industrial-scale solar kerosene production plant in Juelich, Germany, in 2022, and that Swiss will be its first customer. The airline aims to become the world’s first airline to operate flights using solar kerosene in 2023. 1

As much as “solar jet fuel” captures the imagination, it also raises some questions. Can this technology really be called sustainable or even carbon neutral? Does it have a chance of being deployed on a large scale and truly paving the way for zero-emission aviation? Let us take a reality check.

What is “solar kerosene” and how is it made?

Solar kerosene or solar jet fuel has long been the focus of scientists. The idea of producing hydrocarbon fuels from carbon dioxide and sunlight is relatively simple: at high temperatures, CO2 and water dissociate into hydrogen, carbon monoxide and oxygen. The hydrogen and carbon monoxide mixture, known as synthesis gas or “syngas”, can then be converted into liquid hydrocarbons such as petrol or kerosene via a process invented by chemists Franz Fischer and Hans Tropsch in Germany in the mid-1920s – aptly called the Fischer–Tropsch process.

Solar fuel was first produced from scratch in a solar reactor in 2014.2 At the time, chemists made only one jar of kerosene, but the technology has since evolved. The most successful teams in making solar jet fuel were led by Aldo Steinfeld, a professor of renewable energy sources at ETH Zurich, along with colleagues as part of the EU-supported Solar Jet project.

The researchers built and successfully operated a mini solar refinery on the roof of ETH engineering laboratory building in Zurich. “This plant successfully demonstrates the technical feasibility of the whole thermochemical process to convert sunlight and ambient air into drop-in fuels. The system operates stably under real solar conditions and provides a unique platform for further research and development,” said Steinfeld in 2021, assuring that the technology is mature enough to be used in industry.

The research plant in Zurich: The chemical process is powered by solar energy. (Image: ETH Zürich / Alessandro Della Bella)
The research plant in Zurich: The chemical process is powered by solar energy. (Image: ETH Zürich / Alessandro Della Bella)

Synhelion partners with Lufthansa and Swiss

Synhelion – a company that has just partnered with Lufthansa and Swiss, developed its own solar receiver concept and patented it. Solar radiation is reflected by the mirror field, concentrated on the receiver, and converted into 1,500 °C process heat. The generated heat is fed to the thermochemical reactor that produces syngas, which is then processed into fuels.  Excess heat is stored in the thermal energy storage (TES) to enable continuous 24/7 operation.

The company is now planning to launch the world’s first industrial solar fuel production plant. By 2030, it plans to produce 875 million liters of solar fuel per year, enough to cover about half of Switzerland’s jet fuel consumption.

Is “sun-to-liquid” fuel really carbon neutral?

“Sun-to-liquid” fuel can raise questions and doubts. Is it truly carbon neutral? Can it meet global demand for jet fuel?


“Solar fuel is carbon neutral in the sense that all the carbon in the fuel has been taken from the ambient air, so no additional carbon is added to the atmosphere.” Professor Johan Lilliestam, research group leader at the Institute for Advanced Sustainability Studies in Potsdam.

The professor notes that the production of machinery, such as steel elements used in the production of solar fuels, currently produces carbon emissions, but over time, materials production also has a chance to decarbonize. “Emissions will decrease over time and disappear in the long term,” Lilliestam says.

The expert underlines that carbon neutrality does not necessarily mean climate neutrality, as the carbon effect is not the only one heating up the atmosphere.

“Contrails are of particular concern, as some estimates suggest they cause a similar amount of warming as carbon emissions from aviation. It is not clear how large this effect is or how it can be controlled. However, because solar fuels are chemically cleaner than fossil fuels, burning solar fuels causes less soot, resulting in far fewer contrails. So solar fuels are better in that respect as well, probably much better, but there could be a small residual climate effect left over from cloud formation,” he adds.

Will solar kerosene become a standard?

According to Prof. Lilliestam, solar kerosene has the chance to become as popular as any other liquid fuel, and in the long run, it can meet the global demand for sustainable aviation fuels. “The technology is not yet mature, but the collaboration between Synhelion and Swiss/Lufthansa shows that we are close to producing solar fuel for aircraft,” he says.

For solar kerosene to become widely available, large facilities and solar power plants would have to be built. That means significant land areas will be needed for the plants. “It’s probably not possible in densely populated areas like Europe, but it could work well for large-scale production, especially in deserts. The water needed for the process is taken from the air along with the CO2, even in very dry places,” the professor continues.

Experts also point out that placing solar plants in desert areas means there is no competition with agricultural land.3

What’s the cost of solar kerosene?

Another common question is whether solar kerosene will be profitable for the airlines. Synhelion has not disclosed the cost of producing the fuel, but some scientific analysis has suggested that it would cost €1.20 to €2 per liter if produced on an industrial scale.4

“I am pretty sure the cost will come down in the future after deployment and technical development efforts have pushed the learning curve down,” says Prof. Lilliestam. “That will happen, of course, unless unexpected technological problems arise, which can happen with any new technology,” he adds.

The scientist points out that the key to driving down costs is in the adoption of the technology.


“Costs come down in the marketplace, not in the lab. That’s why we are proposing a quota system for solar fuels, possibly specific to each solar fuel technology pathway. The quota needs to be set low to offset the high cost per liter, but still trigger initial factories, and it needs to increase over time – and since costs decrease as volumes increase, the overall cost of such a policy may be manageable.” Professor Johan Lilliestam, research group leader at the Institute for Advanced Sustainability Studies in Potsdam.

More transparency in the aviation industry needed

We believe that the future of aviation can be greener. Solar kerosene brings a new perspective and if it works on a larger scale, it can really be a big step toward carbon-neutral aviation and shipping. Large investments would be needed for this to happen – but so far there are no barriers to making it real.

However, adoption in industrial scale will take time – something we don’t necessarily have the luxury of. Many airlines do not want to just wait for new technologies to maximize their sustainability efforts. More and more of them are working to improve aircraft efficiency, deploying newer aircraft and communicating openly about the emissions generated by flights.

This is where Oncarbon can help, by showing the carbon footprint of a flight compared to other flights on the same route. This way, customers can make an informed travel decision and choose their flight consciously. You can read more about our methodology here and book a free demo to see how our product works. We’ve already been trusted by responsible travel businesses and offer a toolkit you will find nowhere else. Our mission is to make travel more sustainable – and we can help you in achieving that goal, too.

Original cover picture: Luka Slapnikar/Unsplash.


Picture of Joanna Kocik
Joanna Kocik
Passionate about startups and sustainability. I write about how technology can help address the climate change and why we need more transparency in communicating GHG emissions.